Engine control systems require accurate control of exhaust gas recirculation (EGR) for controlling regulated emissions and achieving fuel economy improvements. One type of exhaust gas recirculation system externally recirculates exhaust gas from an exhaust manifold to an intake manifold with a flow control valve placed in the flow path between the exhaust manifold and the intake manifold. Typically, the valve is pneumatically operated and controlled by an electronic engine controller.
One approach to controlling exhaust gas recirculation flow is to use a feedback variable to assure that actual exhaust gas recirculation flow converges to a desired exhaust gas recirculation flow. One method is to use a differential pressure measured across an orifice in the exhaust flow path upstream of the flow control valve. Then, differential pressure can be used to infer the actual exhaust gas recirculation flow. The differential pressure measurement provides a correlation to exhaust flow using the principle that exhaust pressure varies only slightly in the region where EGR is utilized. Using this approach, temperature effects can be accounted for by correlating upstream exhaust manifold temperature to engine operating conditions, or ignored due to relatively small variations. Finally, an error between the actual and desired exhaust gas recirculation flow is used to create a control signal that is sent to the flow control valve. Thus, the system can compensate for the effects of engine and component aging, as well as other errors in the system. Such a system is disclosed in U.S. Pat. No. 5,190,017.
The inventors herein have recognized a disadvantage with the above system when the orifice is placed downstream of the valve. In this configuration, flow from the exhaust travels first through a flow control valve and then through the orifice before entering the intake manifold. In this case, pressure upstream of the orifice (downstream of the valve) varies widely and assumptions made regarding differential pressure and flow are no longer valid. Also, temperature upstream of the orifice (downstream of the valve) is no longer correlated directly to engine operating conditions due to flow expansion in the valve. Thus, there is a significant measurement error when using a differential pressure measurement with a downstream orifice.